In many processing and computing systems, magnetic data storage devices, such as disk drives are utilized for storing data. A typical disk drive includes a spindle motor having a rotor for rotating one or more data disks having data storage surfaces, and an actuator for moving a head carrier arm that supports transducer (read/write) heads, radially across the data disks to write data to or read data from concentric data tracks on the data disk.
In general, a magnetic transducer head is positioned very close to each data storage disk surface by a slider suspended upon an air bearing. The close proximity of the head to the disk surface allows recording of very high resolution data and servo patterns on the disk surface. Servo patterns are typically written with uniform angular spacing of servo sectors and interleaved data sectors or blocks. An example servo pattern includes circumferentially sequential, radially staggered single frequency bursts. Servo patterns provide the disk drive with head position information to enable the actuator, such as a rotary voice coil motor to move the head from starting tracks to destination tracks during random access track seeking operations.
Further, the servo patterns provide the disk drive with head position information to enable the actuator to position and maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data block storage areas in the tracks on the disk surface.
Data transducer heads currently in use employ dual elements. An inductive write element having a relatively wide recording gap is used to write information into the data tracks, and a read element such as a magneto-resistive sensor having a relatively narrow playback gap is used to read information from the data tracks. With this arrangement data track densities equaling and exceeding for example 30,000 tracks per inch are possible.
Conventional servo patterns are written into the servo sectors of each disk using a servo writer at a point in the drive assembly process before the hard disk unit is sealed against particulate contamination from the ambient. Such conventional servo writing method has been largely replaced by a self servo writing method.
In the self servo writing methods, it is necessary to write the servo data precisely at a prescribed position on the disk. The head incorporated in the disk drive still utilizes the two discrete element, i.e., the read-head element and the write-head element. A position offset inevitably exists between these element. The position offset corresponds to the distance between the centerlines of the read and write head elements. Hence, the head-positioning control must be carried out in the self servo writing in accordance with the position offset between the read and write element.
More specifically, the position offset must be measured and the offset must be adjusted in accordance with the position offset measured in order to control the positioning of the head. Some conventional systems have been developed in which the position offset is measured by using a measuring pattern written on the disk. In this method, the measuring pattern is used, determining the distance between the centerline of the read-head element and one end of the write-head element and also the distance between the centerline and the other end of the write-head element. The distance from the centerline of the write-head element is obtained from the average of these distances determines. The position offset is then calculated.
Self servo write process also typically comprises the propagating of tracks radially across the surface as well as phase aligning the servo identification mark. One of the crucial aspects of radial propagation involves keeping the track pitch constant or follow a particular trajectory such that constant radial track pitch is maintained. Keeping the tracks equidistant involves a calibration process. Measuring amplitude of three consecutive tracks A, B and C and computing (A+C)/B or APC while the head is centered on track B gives a relative track pitch that depends on the magnetic write width and read width.
The Self Servo Write process also uses a pre-determined APC profile that is measured with an external positioner system. This profile mimics the profile from a pusher written track with absolute constant radial track pitch. The tracks pitch written using a particular APC profile does not generate an absolute track pitch on a group of files. This is because the APC is a relative measurement of track pitch and the head dimensions vary widely. Since the physical separation of the read and write elements create a positional offset, the offset changes from the inner diameter (ID) to the outer diameter (OD) of the disk. This is because the geometry of the head and the actuator mounting with respect to the disks.
A read write offset (RWOFS) is a function of skew angle of the head. The self servo write process needs more than a particular offset at the start of the propagation and this offset needs to be monotonously increasing function. This feature is designed in to the heads for all self servo written drives. The APC and the PWOFS is a tightly coupled variable during the self servo write process. Controlling the track pitch means maintaining the APC to the target profile by changing the location of servo or changing the RWOFS during the radial propagation.
Various factors influence the quality of track pitch and consequently the generated RWOFS profile for the corresponding tracks. Coercivity, fly-height, low frequency TMR, affects the track pitch. There is also noise in the measurement of APC that forces the controller to have a large latency (low gain controller). These factors produce glitches in the actual APC profile accomplished during the self servo write process which in turn makes the RWOFS curve jittery.
After the self servo write process the product measures the RWOFS curve across the stroke of the actuator (ID to OD) to fit a polynomial. Since the polynomial determined is generally using few measurement points and is of a low order (5th order), it is not enough to account for actual jittery RWOFS profile generated by the self servo write process. This in turn affects the soft error rate to be higher. One of the major issues is a steep slope change of the RWOFS curve at the OD because of coercivity variation of the disk at the corresponding region.
The conventional method of error detection and correction of position offsets is inaccurate. It is therefore desired that the position offset between the read and write head elements be measured with high precision to perform successful self servo write process.